Pairwise mode-locking in dynamically-coupled parametric oscillators

TL;DR

This paper introduces pairwise mode-locking in coupled parametric oscillators, a collective effect similar to laser mode-locking, resulting in broadband, multimode emission with high second-order coherence but no pulsed output.

Contribution

It demonstrates, both experimentally and analytically, the occurrence of pairwise mode-locking in coupled parametric oscillators, a novel collective phenomenon analogous to laser mode-locking.

Findings

Over an octave of bandwidth covered

Approximately 20 resonant mode-locked pairs identified

High second-order coherence achieved

Abstract

Mode locking in lasers is a collective effect, where due to a weak coupling a large number of frequency modes lock their phases to oscillate in unison, forming an ultrashort pulse in time. We demonstrate an analogous collective effect in coupled parametric oscillators, which we term "pairwise mode-locking", where many pairs of modes with twin frequencies (symmetric around the center carrier) oscillate simultaneously with a locked phase-sum, while the phases of individual modes remain undefined. Thus, despite being broadband and multimode, the emission is not pulsed and lacks first-order coherence, while possessing a very high degree of second-order coherence. Our configuration is comprised of two coupled parametric oscillators within identical multimode cavities, where the coupling between the oscillators is modulated in time at the repetition rate of the cavity modes, with some analogy to active mode-locking in lasers. We demonstrate pairwise mode-locking in a radio-frequency (RF) experiment, covering over an octave of bandwidth with approximately 20 resonant mode-locked pairs, filling most of the available bandwidth between DC and the pump frequency. We accompany our experiment with an analytic model that accounts for the properties of the coupled parametric oscillators near threshold.